Recently, a Multiple Input Multiple Output (MIMO) system becomes a focus of attention as a wideband wireless mobile communication technology. The MIMO system refers to a system capable of improving data communication efficiency using multiple antennas.
The MIMO system may be classified into a space multiplexing scheme and a space diversity scheme, depending on whether or not the same data is transmitted. The space multiplexing method refers to a scheme for simultaneously transmitting different data via a plurality of transmission antennas so as to transmit data at a high transmit rate without increasing system bandwidth. The space diversity scheme refers to a scheme for transmitting the same data via a plurality of transmission antennas so as to obtain transmit diversity. Examples of such a space diversity scheme include a space time channel coding scheme.
In addition, the MIMO system may be classified into a single user (SU)-MIMO scheme and a multiple user (MU)-MIMO scheme depending on how many users are allocated to the same time/frequency area. A part having a time/frequency area is referred to as a resource area. One user may be allocated to one resource area in the SU-MIMO scheme and multiple users may be allocated to one resource area in the MU-MIMO scheme. In general, performance of the SU-MIMO scheme is good when the number of users is small and performance of the MU-MIMO scheme is good when the number of users is large.
In addition, the SU-MIMO and the MU-MIMO system may be classified into an open loop scheme and a closed loop scheme depending on whether or not channel information is fed back from a reception side to a transmission side. The open loop scheme includes a Space-Time Trellis Code (STTC) scheme in which a transmission side transmits information in parallel, and a reception side detects a signal by repeatedly using a Zero Forcing (ZF) scheme and a Minimum Mean Square Error (MMSE) scheme and obtains transmit diversity and coding gain using a space domain and Bell Laboratories Layered Space-Time (BLAST) for increasing information amount by the number of transmission antennas, or the like. The closed loop scheme refers to a scheme in which a receiver estimates a radio channel state and transmits the estimated channel state to a transmitter in the form of appropriate feedback information and the transmitter controls channel quality in consideration of the channel state acquired from the feedback information. The closed loop scheme includes a Transmit Antenna Array (TxAA) scheme, or the like.
UEs which perform an MU-MIMO scheme in the same resource area operate on a group-by-group basis. Channel performance may be deteriorated due to co-channel interference between UEs. Accordingly, there is a need for a method for reducing co-channel interference between UEs which perform a MIMO scheme in the same resource area.